An omnidirectional antenna is a type of antenna commonly used in an existing mobile communications device, and the omnidirectional antenna is widely applied to existing networks. In recent years, mobile communication develops towards high-order modulation, broadband, and multiple-input multiple-output technology (MIMO). The multiple-input multiple-output technology (MIMO) is an extremely important development direction. In the multiple-input multiple-output technology, a transmit end and a receive end use multiple transmit antennas and multiple receive antennas, so that signals are transmitted by using multiple antennas of the transmit end and the receive end. Therefore, the multiple-input multiple-output technology can exponentially increase a system capacity and improve spectral efficiency without increasing a spectrum resource. In the MIMO technology, an antenna technology is crucial, especially to a mobile communications device integrating an antenna. The following requirements pose a quite big challenge to antenna design: antenna miniaturization, broadbandization (standing wave broadbandization and pattern broadbandization), isolation between multiple antennas, and a correlation between multiple antennas.
Isolation between antennas and a correlation between antennas are crucial indicators for obtaining a high MIMO gain. A lower correlation between antennas indicates that a higher MIMO gain can be obtained. The isolation between antennas is an important indicator for obtaining a low correlation between antennas. However, because of a miniaturization requirement, it is a quite big challenge to obtain maximum isolation between antennas in a module having a given size.
In addition, a power balance between multiple antennas is also an extremely important aspect. In the multiple-input multiple-output technology, an excessively big power difference between multiple paths usually compromises a MIMO gain. A small tracking difference between patterns of multiple antennas is required for achieving the power balance, and for the omnidirectional antenna, this means that a good roundness (or non-roundness) indicator needs to be achieved. In an existing radio transceiver module integrating multiple antennas, for a purpose of module miniaturization, antenna elements of a PIFA or PILA type are usually selected. For a pattern of a PIFA or PILA, it is usually difficult to achieve a roundness as an independent omnidirectional antenna supporting SISO. This leads to a big tracking difference between patterns of multiple antennas, and affects MIMO performance to an extent.
In an existing common omnidirectional antenna, such as a monopole antenna or a discone antenna with wider bandwidth, a feedpoint and a radiator of the antenna are usually placed in central positions of a ground, and the radiator of the antenna is parallel with a normal line direction of the ground. This perfect rotational symmetry in terms of structure ensures a quite small horizontal fluctuation of a pattern of the antenna, so as to achieve an effect of even coverage.
All existing structures are designed based on a symmetrical structure. When a multi-antenna array is designed by using antenna elements designed based on the symmetrical structure, symmetry of an antenna radiation structure is maintained, but symmetry of the ground cannot be satisfied. This asymmetry usually causes current asymmetry on a carrier surface, and further leads to pattern distortion. A part of design can be maintained relatively good in a narrowband range, but it is quite difficult to achieve relatively wide bandwidth.
In addition, after an omnidirectional antenna element in the prior art is integrated on a carrier, a pattern of an antenna is extremely sensitive to a shape change of the carrier. For example, when the carrier is relatively thin (for example, 0.01λ, where λ is a wavelength corresponding to a minimum operating frequency of the antenna), a roundness of the pattern of the antenna can be ±2.5 dB. However, because the radio transceiver module includes multiple parts, such as a circuit board, a heat sink, and a shield cover, a thickness of a radio transceiver module integrating the antenna is usually greater than 0.01λ. Therefore, when the antenna element in the prior art is integrated on such a module, the roundness of the pattern of the antenna may significantly deteriorate.
A pattern of an antenna located on a corner of the carrier has poor roundness performance because of deterioration of symmetry of a ground around the antenna. As shown in FIG. 1, FIG. 1 is a typical horizontal plane pattern of a broadband antenna that has a Patch-Slot-Pin (PSP) structure and that is mounted on a surface of a square prism carrier. It can be seen from FIG. 1 that depressions of different degrees exist in a shadow area of the figure, and the pattern has poor roundness performance.